According to known technics which are adopted at present, cores of complicated shape or structure i.e. cores whose molding is difficult with a single molding operation are manufactured in such a way that some parts of a core are separately, previously molded, said parts are manually arranged in a core box for molding a complete core, and thereafter the complete core is molded. However, such conventional system troubles the worker so that the molding cycle is not constant and the work is accompanied by danger, and it is known to the skilled in the art that such molding is of poor efficiency.